Electrophotographic apparatuses in recent years have been shifting from office use to personal use, and this has been accompanied by a need for these apparatuses to be smaller and faster, to provide higher image quality, to be maintenance-free, and so on. Accordingly, some of the requirements these apparatuses now must meet include use of a cleaner-less process in which waste toner is recovered in development, without having to clean away waste toner left behind after transfer; the use of a tandem color process with which color images can be output at a high speed; oil-less fixing with which a sharp color image having high gloss and high optical transmissivity can be obtained with no offset even without the use of a fixing oil for preventing offset during fixing; easy maintenance; and low ozone emission. All of these functions must be realized at the same time, and improving toner characteristics, and not only the process, is an important factor.
With a color printer, an image support (hereinafter referred to as a photosensitive member) is charged by corona discharge, using a charger, after which the photosensitive member is irradiated with various colors of latent image in the form of optical signals, thereby forming an electrostatic latent image. This is developed with a first color (such as yellow) toner to visualize the latent image. After this, a transfer member that has been charged to the opposite polarity of the charge of the yellow toner is brought into contact with the photosensitive member, which transfers the yellow toner image formed on the photosensitive member. The photosensitive member is destaticized after cleaning away any toner remaining behind from transfer, which concludes the development and transfer of the first color toner. Then, for other toner colors such as magenta and cyan, the same operation as for yellow toner is repeated, which builds up toner images of various colors on the transfer member and forms a color image. These superimposed toner images then are transferred onto a paper that has been charged to the opposite polarity from that of the toner, and this constitutes a four-pass color process.
There also has been a proposal for a tandem color process in which a primary transfer process first is executed by disposing in a row a plurality of image formation stations having a charger, a photosensitive member, a developing unit, and so forth, and bringing an endless transfer member into contact with the photosensitive member to continuously transfer consecutive colors of toner to the transfer member, so that a multilayer transferred color toner image is formed on the transfer member, and then a secondary transfer process is executed by transferring the multilayer toner image formed on the transfer member all at once to a transfer medium such as a paper or an overhead projector (OHP) sheet, as well as a proposal for a tandem color process involving direct and successive transfer to the paper, OHP sheet, or other transfer medium, without using a transfer member.
In the fixing process, with a color image, the color toner must be melted to mix the colors and increase optical transmissivity. If the toner does not melt sufficiently, light is scattered on the surface or in the interior of the toner image, resulting in a loss of the original color tone of the toner dye and preventing the light from reaching lower layers in overlapping portions, which decreases color reproducibility. Therefore, one of the conditions required for a toner is that it be capable of completely melting and have enough optical transmissivity that does not impair color tone. The need for OHP sheets to have good optical transmissivity has increased as color presentations have become more commonplace. When a color image is obtained, offset occurs when toner adheres to the surface of the fixing roller, so that the fixing roller has to be coated with a large quantity of oil or the like, which complicates handling and the apparatus configuration. Consequently, there is a need for oil-less fixing, in which no oil is used during fixing (discussed below), in order to make apparatuses smaller, easier to maintain, and less expensive. A design in which a wax or other such release agent is added to a binder resin having sharp melting characteristics is starting to be put to use in an effort to accomplish this goal.
However, a problem with a toner such as this is that the toner has the property of being highly cohesive, so that toner image disruption during transfer and the tendency toward poor transfer are likely to be more pronounced, making it difficult to achieve both good transfer and good fixing. Also, in the course of two-component development, toner-spent tends to occur, in which the low-melting point component of the toner adheres to the carrier surface as a result of heat generated by mechanical impact and friction, such as impact and friction between particles, or impact and friction between particles and the developing unit. This diminishes the ability of the carrier to be charged, and shortens the service life of the developer. In an effort to provide a coated carrier having an extended-life, it has been proposed, for example, in Patent Document 1 (see below) that the surface of a carrier core be coated with a resin of a copolymer of a nitrogen-containing fluoroalkyl (meth)acrylate and a vinyl monomer, a copolymer of a fluoroalkyl (meth)acrylate and a nitrogen-containing vinyl monomer, or the like. It is stated in these documents that a coated carrier with a relatively long service life can be obtained by coating the carrier core surface with a solvent-soluble fluorine-containing polymer having imide bonds, or a copolymer of a nitrogen-containing monomer and a fluorinated monomer. Nevertheless, the resin adhesive strength is low at the adhesive boundary with the carrier, and the strength of the resin is too low, so that adequate impact resistance has yet to be obtained. Also, the chargeability of fluorine makes it difficult to negatively charge the toner, so that an adequate charge cannot be imparted to the toner, resulting in image fogging, uneven density, and other such problems.
Patent Document 2, for example, proposes a carrier coated with a silicone resin containing an aminosilane coupling agent in combination with a toner of specific components in an effort to improve the durability of a developer by preventing a decrease in toner charge in atmospheres of high humidity, but these approaches were inadequate in terms of preventing toner-spent.
Patent Document 3 proposes a carrier in which fluorine-substituted alkyl groups have been introduced into the silicone resin of a coating layer, as opposed to positively-chargeable toner. Patent Document 4 proposes a coated carrier containing conductive carbon and a crosslinked fluorine-modified silicone resin, which affords better developing performance in a high speed process, and this performance does not deteriorate over an extended period. This takes advantage of the excellent charging characteristics of a silicone resin, and the fluorine-substituted alkyl groups impart lubricity, partability, water repellency, and other such benefits, make wear, separation, cracking, and the like less likely to occur, and prevent toner-spent. However, not only is the effect unsatisfactory in terms of wear, separation, cracking, and the like, but while suitable charging is obtained with a positively-chargeable toner, when a negatively-chargeable toner is used, the amount of charge is too low, oppositely chargeable toner (positively-chargeable toner) is generated in large quantity, fogging, toner scattering, and other such problems occur, and the product cannot stand up to actual use.
A variety of toner compositions also have been proposed. As is well known, a toner for electrostatic charge development used in electrophotography generally includes a resin component (binder resin), a coloration component composed of a pigment or dye, a plasticizer, a charge control agent, and any necessary additives such as a release agent. A natural or synthetic resin is used, either singly or as a suitable mixture, as the resin component.
The above-mentioned additives are pre-mixed in an appropriate ratio, the mixture is heated and kneaded by thermal melting, and finely pulverized with an air stream collision board, and the resulting fine powder is graded to complete a toner matrix. Chemical polymerization such as emulsion aggregation or suspension polymerization is another way to produce a toner matrix. After this, an additive such as hydrophobic silica is added to the toner matrix to complete the toner. Toner alone is used in single-component development, while a two-component developer is obtained by mixing toner with a carrier composed of magnetic particles.
Patent Document 5 discloses the constitution in which TiO2 whose surface is coated with a compound with a melting point of 40 to 150° C., such as a C13 to C39 saturated fatty acid, a fatty acid ester, or a aliphatic alcohol having at least 15 carbon atoms, is added, and provides a toner whose fluidity and anti-caking properties are improved, and in which cleaning defects hardly occur.
Patent Document 6 discloses a toner containing titanium oxide treated with a fatty acid metal salt (A), and a carrier comprising ferrite particles covered with fluorine resin (B), and provides the effect of enhancing charging stability of the developer and speeding up the start of charging after a new toner is supplied.
Patent Document 7 discloses resin microparticles that have an average particle size of 0.03 to 2.0 μm and whose surfaces have been treated with a fatty acid or a fatty acid derivative, and discloses the effect of obtaining images that are faithful to latent images regardless of the condition of the transfer material, and, in particular, high quality images without partial transfer defects.
Patent Document 8 discloses a toner containing inorganic micropowder subjected to treatment for imparting hydrophobic property through hydrolysis of a fatty acid compound in an aquatic system and inorganic micropowder subjected to treatment for imparting hydrophobic property with silicone oil in an aquatic system, and provides a color toner that is affected less by temperature and humidity, has a stable friction chargeability, and is excellent in producing sharp images without fogging and in durability.
Patent Document 9 discloses a constitution in which magnetic particles whose surfaces have been treated with a fatty acid, a fatty acid metal salt, or a fatty acid ester are added to a polyester resin, in order to obtain a positively-chargeable magnetic toner having a stable image quality even under the condition of high humidity and high temperature.
Patent Document 10 discloses a constitution in which an inorganic compound whose surface has been treated with at least one treatment agent selected from the group consisting of fatty acid metal salts and C20 to C60 alcohols being solid at room temperature is added, and proposes a dry toner for electrostatic charge development that has good fluidity, good cleaning property, excellent environmental stability and durability, and does not cause toner filming on the surface of a photosensitive member, the surface of a carrier used in two-component development, or the surface of a charge imparting member used in one-component development.
Patent Document 11 discloses a constitution in which microparticles with core microparticles coated with a long chain fatty acid metal salt are added to the surface of toner particles. This constitution provides images that have good transfer and do not cause partial transfer defects in characters, while maintaining the image density.
Patent Document 12 discloses a magnetic toner containing hydrophobic silica and a superfine particle titanium oxide powder that is made hydrophobic by surface treatment with a fatty acid salt of aluminum, and provides a magnetic toner having a stable image quality for a long period of time without causing filming on the surface of a photosensitive member.
However, merely adding inorganic microparticles whose surfaces are treated with, for example, a fatty acid does not provide sufficient environmental characteristics, and the treatment amount is limited so that a sufficient charging stability and parting effect cannot be obtained, although the effect is achieved to a certain extent. Furthermore, a constitution in which a large amount of low-melting point wax is blended in a toner for oil-less fixing is not sufficient to maintain good fluidity or to stabilize the quality of developed images.
In Patent Document 13, a non-free fatty acid type of carnauba wax and/or a montan-based ester wax and an oxidized rice wax with an acid value of 10 to 30 are used as a wax serving as a release agent, while a vinyl copolymer having a melting point of 85 to 100° C. that is polymerized in the presence of a natural gas-based Fischer-Tropsch wax is used in Patent Document 14, and Patent Document 15 discloses that a polyhydric alcohol component is polycondensed with a dicarboxylic acid and a trivalent or higher carboxylic acid compound, the average dispersed particle size of the release agent is from 0.1 to 3 μm, the particle size of the additive is from 4 to 200 nm, and the addition is made in an amount of 1 to 5 parts by weight. Patent Document 16 discloses that fixability is enhanced by including a fluorine-modified polyolefin resin such as polypropylene that has been modified with an organofluorine compound such as perfluoro-octyl methacrylate. In Patent Document 17, it is stated that a toner with excellent fixability, offset resistance, and optical transmissivity can be obtained by using a product obtained from a synthetic hydrocarbon wax and an unsaturated polyvalent alkylcarboxylic acid and an alkyl alcohol or amine. In Patent Document 18, it is disclosed that offset resistance during fixing is improved by blending a low-molecular weight polyolefin containing fluorine and having a softening point of 80 to 140° C., which is a molten mixture of polytetrafluoroethylene and a low-molecular weight olefin, and it is stated that this is effective at improving fixability.
The purpose of adding a low-melting point release agent such as polyethylene or polypropylene wax to a resin composition obtained by blending or copolymerizing these high and low molecular weight components is to improve parting from a heat roller during fixing, and thereby increase offset resistance. However, it is difficult to increase the dispersibility of these release agents in a binder resin, oppositely chargeable toner tends to be generated, and fogging occurs in the non-image portions. Filming also tends to occur on the photosensitive member.
A particular problem is the phenomenon whereby the surface of the carrier, which is the toner transport and charging member, is contaminated in the course of using as a two-component developer a toner to which one of these release agents has been added (called toner-spent). Accordingly, there is a decrease in chargeability, as well as a drop in toner transport performance. Furthermore, carrier adhesion tends to be caused, which causes damage to the intermediate transfer member. Therefore, currently the carrier is replaced and discarded after being used for a certain length of time, which drives up the running costs.
Patent Document 1: JP S61-80161A
Patent Document 2: Japanese Patent No. 2,619,439
Patent Document 3: Japanese Patent No. 2,801,507
Patent Document 4: JP 2002-23429 A
Patent Document 5: JP S63-174068 A
Patent document 6: JP H04-452 A
Patent document 7: JP H04-274443A
Patent document 8: JP H05-34984A
Patent document 9: JP H05-72802A
Patent document 10: JP H05-165250A
Patent document 11: JP H05-241367A
Patent document 12: JP H10-161340A
Patent document 13: JP H02-266372A
Patent document 14: JP H09-281748A
Patent document 15: JP2000-214638A
Patent document 16: JP H05-333584A
Patent document 17: JP 2000-10338A
Patent document 18: JP H05-188632A